Sometimes a full size quad can just be too big to fly. Crashing them usually means breaking something so you tend to want to fly them in large areas and very conservatively. For experimentation purposes (and frankly just for fun) a micro quad can be cheaper to build, require less space to fly and generally survive crashes better (as they are generally much lighter). I've been looking to build a 'serious' micro quad for a while. By serious, I mean with a real flight controller and enough power for acrobatic flight.